1. Field of the Invention
The invention relates to a control system, a control device, and a control method of a hybrid vehicular power transmission system that transmits power from an electric speed changing mechanism having a differential mechanism to drive wheels via an engaging device, and is particularly concerned with a technology for improving the durability of the engaging device, and suppressing engagement shock of the engaging device.
2. Description of Related Art
A hybrid vehicle including a differential mechanism having three rotary elements, i.e., a first rotary element to which an engine is coupled, a second rotary element to which a first rotary machine is coupled such that power can be transmitted therebetween, and a third rotary element as an output rotary member to which a second rotary machine is coupled, and an engaging device that transmits power delivered from the differential mechanism to drive wheels, is known. One example of this type of hybrid vehicle is described in Japanese Patent Application Publication No. 2007-002899 (JP 2007-002899 A). The hybrid vehicle of this type includes an electric steplessly speed changing unit and a stepwise speed changing unit, which are arranged in series. In the electric steplessly speed changing unit, the operating state of the differential mechanism is controlled by the first rotary machine, so that the speed ratio is electrically and steplessly or continuously controlled. The stepwise speed changing unit constitutes a power transmission path between an output rotary member of the electric speed changing mechanism and drive wheels. In operation, frictional engaging devices included in the stepwise speed changing unit are selectively engaged and released for changing the speed ratio, so that a selected one of a plurality of gear positions is established.